1. Field of the Invention
The invention relates to a process for obtaining the metallic phase from a dispersed mixture comprised of at least one light metal, in particular aluminum, or comprised of an alloy of this metal or metals and at least one non-metallic phase, constituted by at least one oxide or an oxide compound and/or other compounds of this metal or metals, if need be one or more other substances, e.g. salts or salt mixtures and the like, at higher temperatures, in particular, comprised of dross obtained in processes for producing, processing, and reclaiming aluminum and aluminum alloys when acceleration forces are used. Furthermore, the invention includes a device particularly for carrying out the above-mentioned process, essentially comprised of a metallurgical vessel that can be rotated and tilted.
2. Discussion of Background Information
A melting and in particular a processing of light metals such as aluminum, silicon, magnesium, and this type of alloys in the molten state as well as a reclaiming of these metals by means of melting scrap and materials of that kind in air produce an oxidation and a nitride formation on the surface of the melting pieces and the molten mass, because light metals have a high affinity to oxygen and nitrogen. The oxides produced and non-metallic phases develop with an angular crystalline macrostructure and have a higher specific weight than the metals. The surface tension of fluid light metal and in particular the interfacial surface tension to the high-melting oxides as well as the cavities between the crystals, though, cause the non-metallic phase to float on the fluid metal despite the comparatively higher specific weight.
The surface tension and interfacial surface tension of "fluid metal/solid non-metallic crystals", though, also cause metal particles to be held between the non-metallic crystals, by means of which the slag disposed or formed on the fluid metal bath, a so-called dross, has a high metal content, which can often be up to 80 weight %. As a result, valuable metal is taken out when the dross is removed from the surface of the molten bath, which can produce economic disadvantages when the dross is disposed of and can also cause ecological problems.
Since the beginnings of the technical employment of light metals in large quantities and a use of molten metallurgical processes, great effort has been expended for recovering the metallic phase from the dross or slag that represent a dispersed mixture.
In order to reduce the metal content of the dross, it is possible to add salt to it. This measure is in fact almost completely successful, but for the most part, though, the low-melting salt and the compounds cannot be economically reused and are water soluble as well, and bring great difficulties in handling and disposal.
It has also been proposed (U.S. Pat. No. 4,040,820) that the addition of salts be eliminated and that metal be removed from dross in a horizontal revolving cylindrical furnace that can be heated. By means of a staggered, slow movement of the dross or the dispersed mixture in the furnace, the metal particles should be brought together, united with a fluid metal part, and poured off with it. However, the metal removal gradient of the mixture was low and there were economical problems.
According to U.S. Pat. No. 5,421,850, it has been proposed that the work be likewise carried out without salt in the essentially horizontally disposed rotary furnace, that dross be placed in this furnace, that it be heated to a temperature above the liquidus temperature of the metal by means of plasma heating, and that the metal particles disposed in it be agglomerated and united with one another by means of a mixing motion. A clump formation of the charge should be counteracted by various speeds of the cylindrical furnace, wherein the metallic part formed is removed by means of a tap hole and the residual remainder of dross can be removed by means of de-drossing with a slight inclination of the furnace. The high expenditure and an insufficiently high metal removal gradient of the dispersed mixture are disadvantageous.
GB-2 228 014 A has disclosed a process in which dross or aluminum scrap is heated in a vertical revolving cylindrical furnace by means of plasma energy or a plasma torch, is partially melted, and the fluid metal particles are agglomerated by rotating the furnace, and thus the metal is removed from the dispersed mixture, wherein metallic aluminum and alumina, that is aluminum oxide, are obtained. Air or air that has been enriched with oxygen, nitrogen, and gases of this kind, which react with aluminum, in particular gases that have an oxidizing action, is used as the plasma gas. As a result of an exothermic reaction with the plasma gas, the heating time and melting time of the charge is in fact reduced, however a metal loss occurs due to a melting loss or oxidation and as a result, a dedusting of the exhaust gases is required for the most part and the metal yield is reduced.
Furthermore, processes for obtaining the metal phase from a dispersed mixture have been disclosed in which the fluid metal is coalesced and then solidified. According to WO-A2-93/01321, in a first step, a disintegration of the mixture disposed in an ingot mold at an increased temperature and a coalescing of the fluid metal portion is achieved by centrifuging, after which in a second step, a solidification of the metal into a hollow body is carried out. An improvement and simplification of a process of this kind can be achieved (EP-626 458 A1) if the dispersed mixture is disintegrated at high temperature in one step with the use of an acceleration in an ingot mold, the metallic phase is coalesced and collected in the region of the ingot mold surface, cooled and allowed to solidify.
In order to improve a metal removal from dispersed mixtures of light metal and non-metallic phases, as has been disclosed by AT 400 448 B or EP-548 654 A1, to form a charge, the mixture is introduced into a processing or preparation vessel, the charge is superheated, a uniform temperature distribution is adjusted, and the phases are kept dispersed by movement, after which the homogenized charge is removed from the preparation vessel and placed in a disintegration device. The expenditure, which is high for technical machining reasons, and the complex course of the process, though, can be seen as disadvantageous.
All previously disclosed metal removal processes as well as devices for carrying out a metal removal from dispersed mixtures, in particular from dross, produce an insufficiently high metal yield and/or require a high expenditure for technical machining reasons and/or have a poor thermal efficiency when the metal is put back into a molten mass, and for the most part, cannot sufficiently prevent problems in a removal of residues.